The invention relates to a novel process for preparing cyclobutanone.
It is known that cyclobutanone is obtained by oxidation of cyclobutanol. The processes which have been described employ customary organic oxidants such as various dialkyldioxiranes, chloral, tert-butyl hydroperoxide or substituted or unsubstituted perbenzoic acids; or transition metal or noble metal catalysts including their oxides and oxide complexes (cf., for example, React. Funct. Polym., 29 (2), 101-14, 1996; Can. J. Chem., 62 (9), 1835-9, 1984; Org. Synth., 60, 20-5, 1981; and JACS, 96 (21), 6647-57, 1974).
For an industrial-scale process, the systems chromium(VI) oxide/oxalic acid and ruthenium oxide/sodium periodate are also possible from a process engineering point of view. However, these have the general disadvantage of an excessively high cost of the catalyst system (in the case of ruthenium oxide) or the large amounts of waste and their disposal (in the case of chromium and iodine).
Further known common nonoxidative methods of preparation, e.g. photodecomposition, photooxidation or cyclization, with or without subsequent hydrolysis of derivatives of carbonyl functions, are only suitable on a laboratory scale.
It has been found that cyclobutanone is obtained when cyclobutanol is oxidized by means of alkali metal hypochlorite or alkaline earth metal hypochlorite in the presence of an organic or inorganic acid as solvent.
The process of the present invention surprisingly makes it possible to obtain cyclobutanone in a simple manner in very good yields and in high purity without the cyclobutane ring being expanded to butyrolactone in a BAYER-VILLIGER oxidation as main reaction and opened.
The reaction according to the invention thus has the advantage of a simple, inexpensive and environmentally friendly method of obtaining cyclobutanone, in particular on an industrial scale.
If, for example, the system aqueous sodium hypochlorite/acetic acid is used for the oxidation, the reaction in the process of the invention can be represented by the following scheme: 
Cyclobutanol as starting material is a generally known compound in organic chemistry and is obtainable in a generally known manner, e.g. by reaction/ring expansion of cyclopropyl carbinol with concentrated hydrochloric acid (cf., for example, Org. Synth. 60, 20-25, 1981).
Preferred alkali metal and alkaline earth metal hypochlorites are sodium, potassium and calcium hypochlorites. The hypochlorites are usually used as aqueous solutions. The technical-grade solutions are generally suitable.
Preferred organic acids are alkanoic acids, in particular C1-C4-alkanoic acids such as acetic acid; preferred inorganic acids are mineral acids, in particular hydrochloric acid.
The aqueous sodium hypochlorite/glacial acetic acid system is particularly advantageous.
The reaction temperatures in the oxidation can be varied within a wide range. In general, the oxidation is carried out at from xe2x88x9220xc2x0 C. to +30xc2x0 C., preferably from xe2x88x9220xc2x0 C. to +20xc2x0 C., particularly preferably from xe2x88x9210xc2x0 C. to +10xc2x0 C.
The work-up is carried out in a customary fashion; the pH may optionally be kept in the slightly basic range by addition of alkali metal carbonate or alkali metal hydrogencarbonate, in particular potassium hydrogencarbonate or sodium hydrogencarbonate (cf. the preparative examples).
In one particular embodiment of the process of the invention the starting material cyclobutanol can be prepared from cyclopropyl carbinol in the presence of acids, especially concentrated mineral acids, and the subsequent oxidation to cyclobutanone can be conducted in a kind of xe2x80x9cone-potxe2x80x9d reaction, i.e. in situ without isolating the intermediate or changing the solvent (of the preparative examples).
The preparation of cyclobutanol from cyclopropyl carbinol is known per se. It is generally carried out in water as solvent in the presence of acids, in particular mineral acids such as concentrated hydrochloric acid. Typical reaction temperatures are in the range from 20xc2x0 C. to 120xc2x0 C.; the reaction is generally carried out under reflux conditions.
The cyclobutanone to be prepared by the process of the invention represents a key intermediate of general interest.